Patent Document 1 (JP2006-097898) describes a belt tensioner in which a torsion coil spring, a spring strip and a damper sleeve are provided between a hub and an accommodation casing. In this belt tensioner, one end of the torsion coil spring abuts the accommodating casing and the other end abuts one end of the spring strip. The other end of the spring strip abuts a fixing portion formed on a lid which is firmly connected to the hub, and the damper sleeve is disposed between the spring strip and the accommodation casing. A preload is exerted on the torsion coil spring so as to expand the torsion coil spring in a radial direction, whereby the damper sleeve is pressed against an internal side of the accommodation casing by the radial expansion of the spring strip as a result of exertion of the preload. Then, a braking force is produced by increasing further the pressing force against the damper sleeve.
In addition, Patent Document 2 (JP62-002182B2) discloses a configuration in which a coil spring rotates so as to press a spring support against a boss portion of a rotary member with a predetermined pressing force, whereby a sliding friction is produced between the spring support and the boss portion to thereby attenuate the rotation of the rotary member.
However, in the belt tensioner described in Patent Document 1 above, since the damper sleeve is only simply held between the spring strip and the accommodation casing (a state in which the damper sleeve is fitted in the spring strip) by the radial expansion of the spring strip by the preload, incase the pressing force by the preload is not exerted on the damper sleeve at all times, the damper sleeve slides on an outer circumferential surface of the spring strip when a circumferential force is exerted on the damper sleeve. In addition, although the pressing force against the damper sleeve is adjusted properly by the preload, there is a situation in which the pressing force decreases as a result of the use for a long period of time. In this way, when the pressing force against the damper sleeve decreases, the damper sleeve tends to slide easily with respect to the circumferential direction to thereby reduce the braking force largely.